Studies on the follicle stimulating hormone releasing hormone by radioimmunoassays and by bioassays

The effects of anterior hypothalamic deafferentation on FSH-releasing activity in the intact female hamster

To study the role of the anterior hypothalamic area (AHA) in the control of the estrous (E) rise of follicle stimulating hormone (FSH), female hamsters were subjected to anterior hypothalamic deafferentation (D) or sham deafferentation (S) on 1500 h of proestrus (P). Serum levels of FSH and luteinizing hormone (LH) were measured by radioimmunoassay and FSH and LH releasing activities measured by bioassay in the anterior (AH) and medial basal hypothalamus (MBH) at several time periods during P and E. D did not affect serum levels of LH, whereas there was an attenuation of FSH levels during P and E. D caused an increase in FSH releasing activity in the AH within 1 h, while LH releasing activity was elevated 8 h after D. In animals with D, LH releasing activity in the MBH was elevated during P and was similar to controls during E. FSH releasing activity in the MBH increased during early E and dropped precipitously by 1400 h of E. Collectively, these data demonstrate dichotomous changes in FSH and LH releasing activities and provide further evidence for dual control mechanisms for these two gonadotropins.

Selective modulation of FSH and LH secretion by steroids

Significant divergence between the pattern of FSH and LH secretion has been observed in the ovulatory cycle, after ovariectomy and during puberty. The presence of an FSH-releasing factor, gonadal FSH inhibiting and releasing peptides and changes in the pulsatile pattern of LHRH secretion are among the postulates used to explain the divergent secretion of FSH and LH. Experiments in our laboratory have shown considerable evidence of differential regulation of FSH and LH secretion by steroids in the absence of gonadal regulatory peptides. Natural and synthetic estrogens show significant differences in the suppression of FSH and LH in the ovariectomized rat using a standard uterine response to the estrogen as the end point. In the immature ovariectomized rat treated with a low dose of estradiol that is sufficient for the synthesis of progesterone receptors to ensure progesterone sensitivity, but not large enough to induce estrogen triggered LH surges, progesterone administration resulted in a pattern of LH and FSH secretion similar to that observed on the day of proestrus in the cycling rat. Selective secretion of FSH was induced in the estrogen primed immature rat model by the administration of progesterone metabolite 5 alpha-dihydroporgesterone (5 alpha-DHP) while selective LH secretion was induced by 3 alpha, 5 alpha-tetrahydroprogesterone (3 alpha,5 alpha-THP). The selective secretion of FSH and LH induced by progesterone metabolites was confirmed in the immature female rat primed with PMSG and maintained in constant light. 5 alpha-DHP was also able to induce a greater release of FSH when administered to the adult cycling rat on proestrus. The priming of the pituitary gonadotrope in secreting a high baseline level of FSH or responding to LHRH in releasing a greater amount of FSH appeared to be an important factor in selective FSH release and such priming can be brought about by 5 alpha-DHP in the absence of gonadal regulatory peptides.

Purification of a bioactive FSH-releasing factor (FSHRF)

Before the advent of radioimmunoassay (RIA), FSH-releasing factor (FSHRF) appeared to be separable from LH-releasing hormone (LHRH) by chromatography followed by bioassay for FSH. In this study, we re-examined hypothalamic extracts for the existence of an FSHRF distinct from LHRH, utilizing the Steelman-Pohley bioassay as well as RIA for identification of FSH. Acid extracts of rat hypothalamic fragments were chromatographed on Sephadex G-25. LH- and FSH-releasing activities of each fraction were assessed by bio- and immunoassay of FSH and immunoassay of LH released after incubation with hemipituitaries from adult male rats. The immunoreactive LHRH(IR-LHRH) concentration of each fraction was also measured by RIA. In order to evaluate the FSH-releasing activity of LHRH, three doses of synthetic LHRH were tested and FSH-releasing activity determined by bio- and immunoassay. By RIA, the FSH-releasing activity of each column fraction could be accounted for by IR-LHRH contamination. However, greater FSH-releasing activity than could be predicted by IR-LRH contamination was detected by Steelman-Pohley assay in fractions eluted prior to the LHRH peak in 2 separate fractionations. These fractions from the second fractionation were pooled and eluted from a CMC column with ammonium acetate buffers. Again greater FSH-releasing activity than could be accounted for by IR-LHRH was detected prior to the IR-LHRH peak by Steelman-Pohley assay. These results agree with early work from our laboratory and suggest the presence of a bioactive FSHRF in hypothalamic extracts.